Habitat fragmentation - Practical considerations

Background

As urban development destroys greenspace sites, the species at the remaining sites may become threatened by the impacts of habitat fragmentation.

Habitat fragmentation describes the impact on biodiversity of a reduction in the available habitat combined with an increase in the isolation of habitat patches. Isolation can be in terms of distance, but also concerns how easy it is for species to move through the intervening land uses. The general assumption is that semi-natural habitats are easier for most species to move through than highly modified or urbanised land uses.

Not all species are threatened by habitat fragmentation.

Species that are very sedentary and cannot move very far are affected only by whether or not their particular location is destroyed - some woodland plants are thought to be affected in this way.

Species with very good dispersal are affected only by the total amount of habitat available - good examples are plants with plumes on their seeds and large birds.

Practical considerations

The specific impacts of habitat loss are that population sizes are smaller, or there are fewer populations. The effect of increased isolation is that if an event such as disease kills off a single population, that site cannot be recolonised by individuals from a population nearby.

Many UK species heavily urbanised areas difficult to move through. In particular, hard engineering such as roads and buildings is believed to act as a barrier to species movement. For example dormice (Muscardinus avelliunarius) will not cross large roads. Roads also increase the mortality rates of animal species on the move, including badgers (Meles meles), hedgehogs (Erinaceus europaeus) and toads (Bufo bufo). Concrete buildings and Tarmac don’t provide the food and shelter they need.

Government conservation and planning agencies, from the EU down, are beginning to acknowledge the importance of reducing habitat fragmentation and increasing the connections across landscapes, especially to allow species to move if they need to respond to climate change (Hopkins et al., 2007).

Opportunities

Urban greenspace has an important role to play in reducing habitat fragmentation and retaining some connectivity between patches as development occurs.

Five actions need to be taken in response to habitat fragmentation: in priority order:

Protect existing high-quality wildlife greenspace

Manage and improve degraded greenspace

Restore sites of particular value that have been destroyed (such as wetlands)

Improve the permeability of land use between sites

Create new greenspace.

Existing urban areas

In existing, relatively static urban landscapes, the first, second and fourth priorities are more likely to apply. Features such as footpaths, rivers and canals already offer green corridors in many urban areas. Greenspace that is not suitable breeding habitat for many species may still serve to improve permeability, and therefore movement, between breeding sites. Domestic gardens have an important role to play, and it may be important to communicate to residents the importance of their garden to local wildlife.

New developments

New urban developments can help to counteract fragmentation simply by protecting existing patches of high-quality habitat within them. Making wildlife patches larger is an important measure, as well including new kinds of habitat (such as community woodland on development land that was previously arable) for wildlife, and arranging all greenspace appropriately to encourage species movement.

Services

Forest Research is leading the development and application of landscape ecology tools for combating fragmentation in the UK. Habitat network maps created by the Rural and Urban Landscape Ecology groups have already been implemented in planning and grant applications in south-west England, Wales and Scotland. Habitat network maps can be used to evaluate how connected the existing wildlife patches are, and to target where to place new patches during site development.

In tandem, Forest Research is performing research to inform and validate its recapture and genetic data can all inform-tools and models. Radiotracking, mark recapture models and direct observation can all provide important information us about how species move through the landscape.

Local authorities wishing to include habitat fragmentation-reduction measures into their spatial planning can benefit from an integrated habitat network modelling and advisory service, which includes long-term data set maintenance. We are already providing this to local authorities in Scotland, for example in the Lothians and Clyde Valley.

Simpler analyses can also be employed to evaluate plans, projects or incentives. For example, on the Isle of Wight, landscape metrics such as average edge ratio were employed to assess the effectiveness of a-woodland size and area spatial targeting scheme for woodland grants. Such analyses could easily be used to compare different greenspace scenarios in urban development plans.

Case study

Great crested newt populations

Populations of the great crested newt (Triturus cristatus) have declined, in part due to loss of ponds to development, agricultural change and agrochemicals, but also due to the ‘degradation, loss and fragmentation’ of terrestrial habitats (UK Biodiversity Action Plan). A significant proportion of the UK population are found around towns in north-east Wales, in a line along the A55 from Wrexham to Rhyl. They occupy small ponds, particularly in quarries, and are sometimes found in people’s gardens.

The modelling carried out for this project at Forest Research produced maps of functional habitat networks, outlining which ponds were likely to be close enough to one another for great crested newts to move between them. The networks took account of both distance and the composition of the land between ponds.

We then used data (supplied by the Amphibian and Reptile Conservation) to examine the importance of the ponds and networks to the overall population, and plotted paths between ponds to show the shortest route, accounting for barriers. These paths can be used to inform developers and planners of how important a site may be for the movement of great crested newts.

The Buckley Claypits and Halkyn mountain quarry areas contained the most newts, and effort could be focused on increasing links between, and outwards from, these two population centres. The A494 and A5119 are the greatest barriers to this potential newt corridor, although it is possible that newts use a the disused railway underpass. Further urbanisation around the north-east fringe of Buckley would also hinder connectivity.